"Based on the information available to us, we see no indications that Trump Tower was the subject of surveillance by any element of the United States government either before or after Election Day 2016," read a statement by Republican Chairman Richard Burr and Senator Mark Warner, the committee's Democratic vice chairman.

"There was no such wiretap activity mounted against the president-elect at the time, or as a candidate or against his campaign,"Clapper said.

Under U.S. law, a federal court would have to have found probable cause that the target of the surveillance is an "agent of a foreign power" in order to approve a warrant authorizing electronic surveillance of Trump Tower.

Asked whether there was such a court order, Clapper said in the March 5 interview, "I can deny it."

Former Obama adviser Ben Rhodes also tweeted a sharp rejection of the allegations.

Democrats accused Trump of trying to distract at the time from the rising controversy about possible ties to Russia. His administration has come under pressure from FBI and congressional investigations into contacts between members of his campaign team and Russian officials.